The present invention relates to a steering system, and more particularly to a steering system wherein the rigidity of a transmission system by which a steering effort applied to a steering wheel is transferred to road wheels is made variable, thereby to enhance the control performance of a vehicle.
Known prior art is a steering system described on Pages 137 to 141 of NISSAN SERVICE MANUAL No. 451 "NISSAN LAUREL SPIRIT" issued by Nissan Motor Company Limited in January 1972. As shown in FIGS. 1a and 1b, this known steering system includes a rack-and-pinion type steering mechanism composed of a pinion 12 connected via a steering shaft 11 to a steering wheel (not illustrated) and a rack 13 extending transversely of a vehicle and meshing with the pinion 12. The steering mechanism 14 is received in a gear housing 17 supported on a vehicle body 15 via a mounting apparatus 16. The rack 13 is slidably received in the gear housing 17 in an axial direction and has both ends thereof connected to change direction road wheels (not shown) via tie rods 18 and 18', respectively. With this steering gear mechanism 14, the pinion 12 rotates when the steering wheel is manipulated, causing the rack 13 to move in its axial direction, and this movement of the rack 13 transfers a torque applied to the steering wheel and an angular displacement of the steering wheel to the road wheels to cause them to change their directions.
The mounting apparatus 16 includes brackets 19a and 19b fixedly secured to the vehicle body 15, and tubular gear housing mount insulators 20a and 20b of a rubber-like elastic body which are mounted between the brackets 19a, 19b and the gear housing 17. The mounting apparatus 16 functions to prevent transmission of vibrations due to bumps from the road to the steering wheel by elastically supporting the gear housing 17.
It is widely known that the rigidity of the steering effort transmission system, including a steering shaft 11, via which a steering effort applied to the mounting apparatus 16 for the steering gear 14 and the steering wheel is transfered to the change direction road wheels which is called "steering rigidity" has a great effect on turning characteristics, control and stability performance, and vibrations transmission characteristics of the vehicle. If the steering rigidity is low, the turning characteristics of the vehicle show an apparent understeer tendency because the steering effort applied to the steering wheel is not smoothly transferred, lowering a steering response, and a portion of displacement of the steering wheel is absorbed by deformation of the transmission system, decreasing a change direction angle of each of the road wheels (change-direction angle), although kicks and shimmies due to transmission of vibrations are reduced. If, on the contrary, the steering rigidity is high, the turning characteristics of the vehicle show an apparent neutral and/or oversteer tendency where straightforward driving ability and steering response of the vehicle are enhanced although kicks and shimmies are easy to be presented to the vehicle body. Therefore, the steering rigidity required for the steering system differs with variation in running state of the vehicle and the rate of displacement of the steering wheel. Therefore, it has been demanded for the purpose of enhancing running stability of the vehicle and improving manipulation feel of the steering wheel that the steering rigidity be varied depending upon running state of the vehicle and rate of displacement of the steering wheel.
However, the previously described prior art steering system fails to meet these demands because the steering rigidity is singularly determined by the gear housing mount insulators 20a and 20b of the mounting apparatus 16 and cannot be varied with variation in running state of the vehicle and the like.
One conceivable measure to vary the steering rigidity is to provide a movement limiting arrangement for limiting the displacement of the gear housing 17 beyond a predetermined range in order to vary the steering rigidity in non-linear manner. However, the variation in the rigidity obtained with this measure is not satisfactory in enhancing the control performance of the vehicle because the displacement of the gear housing 17 shows a predetermined steering rigidity that is determined by the gear housing mount insulators 20a and 20b until its movement is limited by the limiting arrangement.